Development of macromorphology in the phenylene-bridged network

As described in Chapter 1.3.3, the macroporous network develops due to a phase separation into silica-surfactant rich and solvent-rich areas. At some point the system is frozen in by the sol-gel transition. The onset between these two processes as well as the concurring mesostructure evolution determines the final macromorphology of the network.

For the investigated phenylene-bridged systems, bPh490-A to C, the colour changed from transparent to turbid (tps in Table 3.6) shortly before long-range order became visible in the SAXS measurements. In contrast to the systems investigated at Elettra (Chapter 3.4.2) gelation was observed after phase separation and mesostructure formation. The SAXS patterns of the wet gels prepared with different Si/10^-2 M HCl ratios after one week of aging at 40 °C are shown in the topmost curves in Figure 3.4.15. In all three systems the peaks relate to a hexagonal ordering of the mesopores with a d10-spacing of ~11.9 nm. A

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

40 60 80 100 120

200 250 300 350 650 700 750 800 850 900

System A System B System C

Coherence length / nm

Time / min

1 week

lower limit for the coherence length L was estimated from the first reflections (sample detector distance of 10 m at ID02) according to Eq. 2.13.

In samples bPh490-A and bP490-B the coherence length for the liquid crystalline domains at the start of the kinetic measurements was found to be approximately 55-70 nm. The broadness of the peaks increases continuously until the point, where the periodic mesostructure starts to form. This transition results in large domains with hexagonally ordered cylindrical mesopores.

Figure 3.4.21. Calculated coherence lengths during the evolution of the mesostructured domains and the macroporous network taken from the in-situ data starting with the first appearance of the Bragg reflections, plus the values derived from the wet, aged gels (after 1 week).

From the estimated size of the coherent regions at the point of mesostructure formation, one can see that the domains exhibiting periodical arrangement are already very large at or shortly after phase transition. In system bPh490-A, where mesostructure evolution and sol-gel transition take place more slowly compared to the more diluted systems, the estimated domain size is approximately 200 nm at the beginning of structure formation and increases to as much as 380 nm during the following 30 min. In system bPh490-B and bPh490-C the contributing regions measure approximately 750 nm right from the start. The coherence length increases during the course of synthesis. This is shown in Figure 3.4.21. Fitting was performed using Mathematica. Surprisingly, we find much lower values for L in case of the aged gels. This is especially true for the system bPh490-B, where the loss in domain size is as large as 67%. In system bPh490-C a relatively small loss in coherence length (~15%) is found, which can be explained by shrinkage of the periodic structure from a

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

System A System B System C

increasing solvent concentration in the starting solution

2 μm 2 μm

0.2 μm

0.2 μm 0.2 μm0.2 μm 0.1 μm0.1 μm

lattice parameter a=15.6 nm, 55 min after mixing, down to a=13.7 nm (12%) after one week of aging. The loss of domain size in the aged gels is probably caused by micro-cracks or boundary regions, respectively, and is most pronounced in system bPh490-A and bPh490-B, where the final network consists of smaller parts and therefore has more contact areas between different domains.

Macromorphology of the Resulting Dried Gels

Concerning the macrostructure we found in the SEM-images spherical to oblate structures with dimensions of 250 nm to 500 nm for bPh490-A, and disc like structures with dimensions of approximately 450 nm thickness and disc diameters up to 2 μm for bPh490-B. For the even more diluted composition in case of bPh490-C, the thickness of the discs increases to approximately 550 nm and the diameters of the platelets become larger up to 5 μm. Scanning and transmission electron microscopy images of the macromorphology of the aged and dried gels are shown in Figure 3.4.22.

Figure 3.4.22. SEM and the corresponding TEM images of the three investigated phenylene-bridged systems.

With increasing dilution, the interconnected parts building up the network become more and more disc-shaped. As the macroscopic dimensions (diameter of the discs) increase at larger dilution, the FWHM of the Bragg d10 reflection becomes smaller, in accordance with an increase in the mean size of coherent regions.

The morphologies of the mesoporous materials are developed after evolution of silica/surfactant-rich regions and are mainly influenced by the competition between the free energy of mesostructure self-assembly (ΔG) and the colloidal surface free energy (F) [33]. When phase separation occurs quickly, ΔG is dominant and the macrostructure of the mesoporous material is developed together with the formation of the mesostructure.

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

Therefore, resulting morphologies will have smaller curvatures, resulting in e.g. rods or platelets as found for the systems investigated in this work. Experimental conditions that increase the induction time for the phase separation such as lower temperature or smaller solvent content lead to morphologies with higher curvature (small surface) such as spheres (bPh490-A). F enforces the optimal trade-off between volume and surface of the resulting particles. Whether the final morphology is more spherical or more regular is furthermore governed by the balance between the polymerization of the inorganic species and the rate of the mesostructure formation [165]. Nevertheless, an internal structure with long range order, such as 2D hexagonal mesopores, will force the particles to adopt a corresponding form.

A further requirement for the formation of a highly ordered mesostructure is that the polymerization rate has to be slow compared to the self-assembly on the molecular level.

For the hybrid networks derived from the phenylene-bridged precursor, the macroscopic network was found to consist of platelets with resemblance to hexagonal prisms. From sample bPh490 A to sample C the ratio between ‘diameter’ and thickness increased with increasing dilution.

In-situ SAXS observations on the macromorphology development

In principle, the process of network formation should be visible in the low-q-range (USAXS). For the q-range (0.018<q<0.6 nm^-1) employed in the experiments on bPhGMS-systems at ESRF, no growth of macroscopic structures could be observed. This can be explained by a spontaneous phase separation into regions larger than 100 nm, respectively a spinodal phase separation, where suddenly a sharp interface/contrast is achieved. For the accessible q-range in the experiments, only objects with dimensions up to approximately 100 nm are identifiable through their characteristic Guinier slope. This was already shown in Chapter 2.3.1 for simple spherical, cylindrical and disc-shaped scattering objects. For polydisperse systems, the minima and maxima in the scattering curves are smeared out beyond recognition leaving only the characteristic slope in the low q-region and the following q^-4-decrease in the Porod region. This was demonstrated in Figure 2.3.2 (b) for discs (Eq. 2.7) with a mean thickness of 300 nm. Polydispersity is taken into account by assuming a Gaussian size distribution in Eq. 2.8.

Obviously the q-range observed in the experiments, did show a q^-4-behaviour, but did not include the Guinier regime of the macroporous network. Therefore, it is at the moment not possible to make any statements on the macroscopic structures during the formation of the macroporous network. Nevertheless, due to the emergence of minima and maxima in the low q-region during the formation process (pointing towards a very narrow size

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

distribution), it is possible to derive some conclusions, such as the dimensions of the scattering objects and their polydispersity.

The macrostructure of the dried gels is known from SEM and TEM investigations (see Figure 3.4.22). In systems bPh490-B and bPh490-C the resulting interconnected particles forming the macroporous network are very similar to discs, for which the radius, R, (or diameter, respectively) is large compared to the height, H. Therefore, the analytical expression for the form factor of bilayers (Eq. 2.7) was used for fitting [166], assuming that R is large and does not contribute. A Gaussian distribution was used to describe the polydispersity of thickness, H, and thus damping of the oscillations in the experimentally obtained scattering data of the platelets (see Eq. 2.8):

( )

D is the mean thickness of the disc-shaped object and σ is the standard deviation.

The resulting values for thickness H are given in Table 3.7 for the aged gels, together with their size and the time of their first appearance during synthesis. During the kinetic measurements, periodic features in the low q-range of the SAXS data can be seen approximately 10 min after evolution of the sharp Bragg reflections of the hexagonally ordered cylindrical mesopores, but only in bPh490-B and bPh490-C. Thickness (H), resulting from the fit according to Eq. 3.4, is already large with 585 nm or 593 nm for the two systems, respectively. The values obtained for bPh490-A have to be handled with care, because the periodic minima and maxima are very weak compared to the other two compositions. Furthermore, the structure is more spherical than disc-shaped. Periodic minima and maxima are most pronounced in the highly diluted system (-C), which consists of large platelets as already shown in the SEM images.

The estimation values for thickness are only rough due to idealistic assumptions of the model and do not fully agree with the dimensions found in the SEM and TEM images (H=450-550 nm) of the dried gels. In accordance to the results of the SEM/TEM images, the platelets in system -C are thicker than in -B. The intensity increases upon aging, whereas σ becomes smaller. Deviation between fit results and the values evaluated from the SEM/TEM images of the final dried gels may furthermore be caused by shrinkage during the drying process. Larger differences are found for system -B due to the applied model, which is not fully accounted for (R~H).

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0

0,02 0,03 0,04 0,05 0,06

0,1

Figure 3.4.23. In the low q-region of the scattering curves, periodic minima and maxima were observed for the aged gels. This feature is most pronounced for bPh490-C and was assigned to the macroscopic parts forming the inorganic network. (a) Low q region of the aged, wet organo-bridged gels with periodic minima and maxima observable for system bPh490-C and bPh490-B. (b) Scattering curve of system bPh490-C with the corresponding fitted curve. For the dashed line a*q^-n was subtracted from the original data to enhance the visibility of the “wiggles”. The overlaid triangular points correspond to the form factor for bilayers with a very small thickness-distribution (σ=0.1 nm). (c) Both curves from (b) in logarithmic scale.

Table 3.7. Parameters obtained by fitting the form factor of discs to the aged, as well as, just synthesized P123/bPhGMS/10^-2 M HCl systems (bPh490 A to C). The radius of the discs, with a thickness (H) and a standard deviation (σ), was assumed to be infinite. The appearance of the periodic features in the in-situ measurements was observed after the mesoscopic phase transition.

*) Results for the fit-parameters for system A have to be accepted with reservation since no distinct minima and maxima are observable. Also the macrostructure of the system does not exhibit the distinctive disc-shape.

Aged gels Evolution during sol-gel process System Amplitude

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

The appearance of periodical features at small q observed in the synthesis of phenylene-bridged gels indicates a high monodispersity of the evolving macroporous network components. This effect is most pronounced in the diluted system bPh490-C, where the time between the evolution of first Bragg peaks of the periodic mesostructure and the observed gel time is longest.